I don't think Redford's appearance is going to win new fans over that weren't there before. Instead, Redford's adding gravitas to the cast which is going to make movie critics and other actors, and Hollywood in general, sit up and take note that this movie is intended to carry a *lot* more weight than the first one. This has just become Serious Business in Hollywood. In much the same way that Branagh's name made all the Shakespearean theater crowd recognize that Thor was going to be more than just a popcorn flick.

It's sort of like how they got Tommy Lee Jones for CA: TFA and it added a sense of credibility to it. After the Avengers, I think Marvel Studios really has a lot of juice now and actors now have to seriously consider them a major Hollywood player.